It is known to provide turbine moving blades with a protective layer so that they have a prolonged service life when they are in operation in a gas turbine. In this context, the protective layer applied to the turbine moving blade manufactured by casting is often a corrosion protection layer of the type MCrAlY. The protective layer is in this case applied in that region of the blade surface which is exposed to the hot gas when the gas turbine is in operation. This region comprises both the blade leaf and the platform of the turbine moving blade, the blade leaf being integrally formed on said platform. Moreover, in addition to the corrosion protection layer, a heat insulation layer may be applied in the abovementioned region, in order to keep the introduction of heat from the hot gas into the basic material of the turbine moving blade as low as possible.
It is known, furthermore, that turbine moving blades are exposed to the excitation of oscillations when the gas turbine is in operation. Excitation to oscillation occurs because of the rotation of the rotor to which the turbine moving blades are fastened. A further contribution to the excitation of oscillations in the blade leaves of the turbine moving blades is made by the hot gas which impinges onto them. Since the blade leaves of the turbine moving blades rotate downstream of a rim of turbine guide blades, as seen in the direction of flow of the hot gas, these are excited to oscillate by cyclically impinging hot gas. It is therefore necessary that each turbine moving blade has a sufficiently high characteristic frequency to ensure that neither the excitation to oscillation emanating from the rotor rotational speed nor that emanating from the hot gas, with respective exciting frequencies, leads to an inadmissibly high oscillation of the blade leaf. Accordingly, in the prior art, the turbine moving blades are designed in such a way that their characteristic frequency deviates from the exciting frequencies of the stationary gas turbine. Care is therefore taken, in the development of the turbine moving blade, to ensure that the finished turbine moving blade, overall, satisfies the requirements with regard to natural resonance.
In the process for manufacturing the turbine moving blade, therefore, there is provision for checking the oscillation properties of each individual turbine moving blade. Insofar as the turbine moving blade does not fulfill the stipulated frequency values in terms of characteristic frequency, it has to be rejected or manipulated by means of suitable measures in such a way that it is then suitable for operation and fulfills the requirements as to characteristic frequency. So that turbine moving blades which are not intended for use in the gas turbine solely because of their oscillation property can still be employed, it is known from U.S. Pat. No. 4,097,192 to introduce a recess on the end face of the blade leaf of the turbine moving blade, with the result that the mass of the turbine moving blade at its free oscillatory end can be reduced. By the mass of the turbine moving blade being reduced, the oscillation property is influenced positively. Its characteristic frequency can be shifted toward higher characteristic frequencies by the removal of the mass, in particular at its outer end.
Moreover, WO2003/06260A1 discloses a method for changing the frequency of moving blades which are already ready for use. According to this, to change the frequency, a metallic covering is applied to the blade leaf in the region of the blade leaf tip, the thickness of which covering tapers continuously at the outlet edge and in the radial direction toward the blade foot. The disadvantage of this, however, is that the aerodynamics of the moving blade are consequently also modified.
Moreover, it is known that measures for prolonging the service life are carried out on turbine moving blades previously used in gas turbines. These measures comprise, on the one hand, the elimination of cracks which have occurred during operation and, on the other hand, the renewal of the protective layers provided on the turbine moving blades.